The invention relates to network antennas of the radial waveguide type with linear slots, known as RLSAs (Radial Line Slot Antennas), that are intended to be more particularly used in satellite communications systems. In these communications systems, transmission to the satellite and reception from the satellite take place along two orthogonal (circular or linear) polarizations, respectively, although the transmission and reception frequency bands are generally different. This decoupling between the two links—the uplink and the downlink—of the communications system is enhanced the better the isolation between the two orthogonal polarizations in the network antenna. The identical approach of two orthogonal polarizations is used in wireless terrestrial communications systems, known as LMDSs (Local Multipoint Distribution Systems), that operate in the 40 GHz millimetric bands.
A network antenna of the RLSA type having a feed structure that allows the antenna to be excited in two orthogonal linear polarizations is known from the document by F. J. Boebels & K. C. Kelly entitled “Arbitrary Polarization From Annular Slot Planar Antennas” published in IRE TRANSACTIONS ON ANTENNAS AND PROPAGATION, July 1961, pages 342–349. The feed structure for this antenna consists of two radial cavities, one placed above the other inside the antenna, each cavity being excited by a circular waveguide placed at the centre of one of the two faces of the antenna. The two ports of the feed structure are thus placed on either side of the antenna, this having the effect of creating masking and perturbation regions at the front of the antenna and therefore downgrading the radiation characteristics of the latter.